Normally, in a case where (a) a receiver device includes (i) an air conduction receiver which allows a user to receive a phone call by air conduction and (ii) a bone conduction receiver which allows the user to receive the phone call by bone conduction and (b) the user uses the bone conduction receiver, the user listens to a sound by bringing the receiver device into contact with a user's ear or a part of a vicinity of the user's ear. In this case, since the user listens to the sound directly from a contact part of the user's ear or the part of the vicinity of the user's ear which part is in contact with the receiver device, it is possible to correctly hear the sound irrespective of external environment, even in a case where an acoustic pressure is low.
On the other hand, in a case where the user uses the air conduction receiver of the receiver device, the user does not listen to the sound directly from the contact part of the user's ear or the part of the vicinity of the user's ear which part is in contact with the receiver device, but listens to the sound by spatial vibration. Therefore, the sound is likely to be affected by the external environment. In a case where an external sound is loud, it may not be possible to listen to the sound unless the acoustic pressure is increased.
Furthermore, a sound audible by bone conduction is generally different, in characteristics such as a frequency characteristic and clarity (acoustic parameter), from a sound audible by air conduction. Therefore, it is necessary to separately and properly set the bone conduction receiver, depending on characteristics of a sound audible by bone conduction, so that the sound is easily audible from the bone conduction receiver. Similarly, it is necessary to separately and properly set the air conduction receiver, depending on characteristics of a sound audible by air conduction, so that the sound is easily audible from the air conduction receiver.
Various techniques have been thus suggested which cause a sound to be easily audible from each of the air conduction receiver and the bone conduction receiver.
For example, Patent Literature 1 discloses a technique of switching between a bone conduction filter characteristic and an air conduction filter characteristic, depending on volume, so that a sound is easily audible from any one of a bone conduction receiver and an air conduction receiver.
Patent Literature 2 discloses a patent related to how to use a terminal in which a bone conduction speaker and an air conduction speaker are realized by an identical element. This is a technique based on a characteristic of a bone conduction terminal such that audibility varies depending on a pressure of the bone conduction terminal to an ear, and is a technique of changing a ratio between a bone conduction sound and an air conduction sound by manually changing the pressure to bone.
Further, Patent Literature 3 discloses a technique of (i) providing, to a terminal, a bone conduction receiver, an air conduction receiver, and a pressure sensor and (ii) changing a ratio between a sound audible from the bone conduction receiver and a sound audible from the air conduction receiver depending on a pressure.